The ArachnoidThe arachnoid is a delicate membrane enveloping the brain and medulla spinalis and lying between the pia mater internally and the dura mater externally; it is separated from the pia mater by the subarachnoid cavity, which is filled with cerebrospinal fluid.

Last month, Sports Illustrated chronicled Meyer's coaching career and reported that he suffered from persistent headaches caused by an arachnoid cyst that becomes inflamed by stress, rage and excitement.